D.C. Sessions wrote:>>How do you propose to keep several legs ALL less than 12mm long?>(at least 60ps/cm, risetime of less than 300ps, line length>Tr/4 => 1.25cm) Note that this is a foolishly optimistic>calculation.>

D.C.,

Right, of course, with psec rise times. I'm more used to a couple of ns of
rise time, such as which Chris was asking about. In that case, 4 to 6 inch
legs of a star may be possible, but only worth the effort if the equivalent
length sum of a daisy-chain adds undesirable delay or skew.

I guess the question I'm posing is: Would, in that case, reflections then
add, acceptably, while the signal is still> transitioning? Or would this lead to too large an amplitude due to multiple> simultaneous reflections from each leg?

Shayle

>> At 10:58 AM 3/29/99 -0800, you wrote:>> >>Chris Bobek wrote:>> >>> >>> >>I have an address bus that is 75ohm, and 19" long. There are 8 loads>that>> >>are daisy-chained along the trace. The edge from the driver (which>> >>is at one end) is 3ns.>> >>> >>I was wondering if I would be better off with a star pattern, where >each>> leg is short (less than 4"). Would this work and not require>> >>any termination?>> >>> >>> >>Thank you very much,>> >>> >>> >Star patterns are a nightmare. There are so many reflections in>> >the system that it easily becomes unstable. If the star is not>> >perfectly balanced your signal integrity will degrade rapidly.>> >Just the process variations for each agent can be enough to degrade>> >your signal integrity to an unacceptable level. Furthermore, if>> >you have a GTL-like bus a star wont work because it would require>> >too many pull-up resistors. Star topologies can be made to work,>> >but not without a lot of pain. They should be avoided for a high>> >speed bus unless all other options have failed.>> >>> >**** To unsubscribe from si-list: send e-mail to>> majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE>> si-list, for more help, put HELP. si-list archives are accessible at>> http://www.qsl.net/wb6tpu/si-list ****>> >>> >>> >> **** To unsubscribe from si-list: send e-mail to
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